Consider using Poison Cleansing Totem instead. It has the same mana cost or less if you have Totemic Mastery and will retry every 5 seconds for five minutes, however your target must be in your party, within 30 yards, and you will lose any other water totem you have out. If more than one person in your group are poisoned, it is usually faster and cheaper to use Poison Cleansing Totem and then immediately replace it with the water totem you had up before. The reduced global cooldown on totems means that this 'totem switch' will only take two seconds, whereas casting Cure Poison twice takes three seconds.